1) Field
Embodiments of the present invention generally relate to microelectronic fabrication, and more particularly to methods of controlling a process parameter in a process recipe performed by a semiconductor manufacturing apparatus.
2) Description of Related Art
Today, the majority of semiconductor fabrication processes are performed with single-substrate manufacturing equipment. Improved process uniformity and control has generally outweighed the throughput reduction associated with such serial processing as compared to parallel processing of batch equipment. With the continuing reduction in the dimensions and film thicknesses formed in a given manufacturing process, the duration of each particular value-added step, such as an etch or deposition, in a fabrication process has steadily become shorter. However, each value-added step typically requires a certain amount of non-value-added process time overhead including such things as substrate handling and stability steps. To reduce handling overhead, many process recipes now combine a number of value-added process steps in sequence. However, such in-situ recipes still typically require a stability step of a finite duration to allow the manufacturing process parameters to be switched from a setpoint used in a first value-added operation to another parameter setpoint for a subsequent value-added operation. Depending on the duration of each stability step, the cumulative overhead for a given process may become a significant fraction of the total process time for a substrate or workpiece in a single-substrate manufacturing tool.
For example, FIG. 1 depicts a typical process parameter response during a fabrication process recipe sequence. In this example, the process parameter is the process temperature of a substrate holder, commonly called a chuck. The chuck temperature is controlled to heat the substrate to various controlled temperatures during the process recipe. As shown, the recipe begins with a first stability step, “Stab1” at time “00” on the x-axis at which time the actual chuck temperature, “Inner_Temp,” is approximately 32° C. as controlled to the chuck temperature setpoint, “Inner_Setpoint.” The duration of “Stab1” is a relatively short 8 seconds, after which the value-added “Step1” is performed until a total process recipe time of 30 seconds has elapsed. “Stab1” only needs to be a few seconds to stabilize process controllers having short response times, such as mass flow controllers (MFC). At time “30” on the x-axis, “Stab2” begins, during which the “Inner_Temp” is controlled to a second “Inner_Setpoint” temperature of 45° C. As depicted in the graph, the actual chuck temperature requires approximately 30 seconds to heat from the 32° C. setpoint of Step1 to the 45° C. required in the second value-added step, “Step2.” Then, “Step2” is performed for approximately 35 seconds, until another 25 second stability step, “Stab3” is required to reach a 52° C. chuck temperature. Following “Stab3,” the value-added step “Step3” is performed for 15 seconds. Conventionally, a linear controller, such as a PID controller is employed for each of the “Inner_Setpoint” values required by a particular recipe.
As evident from the graph in FIG. 1, the cumulative non-value added stability time represents a significant amount of overhead to the total process recipe time. In this specific example, this non-value-added time accounts for approximately 47%, or about half, of the total process time. The overhead incurred will likely become a greater fraction as value-added step duration continues to decrease with device scaling.
Similarly, the fraction of non-value-added time will also increase as recipes with a greater number of steps requiring process parameter setpoints spanning a wide setpoint range are performed in-situ (within a single manufacturing apparatus) rather than in separately tuned systems. As the setpoint range gets wider, non-linearity in the system renders the conventional linear PID control algorithm inadequate and the response time for the extremes in the setpoint range become intolerable.
An adaptive control architecture would advantageously reduce the process overhead.